This invention relates to a pressure fixing device and in particular to a device for the pressure fixing of a toner image on an image record carrier in printing and copying systems. In printing and copying systems, there is employed electrophotographic recording apparatus which usually contains a photosensitive drum. An electrostatic latent image is developed by a well-known process on the surface of said drum. By applying a toner to said latent image, there is formed a toner image which is later fixed by means of a fixing device on a record carrier after said image has been transferred thereon.
For fixing devices of known types, heated rolls are utilized. Such devices have the disadvantage that a certain amount of heat-up time is necessary and are susceptible to a risk of overheating which makes it difficult to ensure the safety for fixing operations. As a result, pressure fixing devices have recently been introduced to a significant extent for the fixing process. In the case of pressure fixing devices, it is necessary to apply pressing forces uniformly on a record carrier to fix a toner image thereon.
A known pressure fixing device having a pair of fixing rolls arranged axially in parallel to each other has the disadvantage that the device becomes large-sized if the diameters of the fixing rolls are increased in order to ensure a pressing force to be uniformly applied even as to different widths of a record carrier. On the contrary, it is a disadvantage that an uniform pressing force can not be obtained if said diameters are reduced, due to a bending deformation of said rolls under the load.
There is also known a pressure fixing device in which, for the purpose of ensuring a uniform pressing force, separate auxiliary pressure rollers are provided. Such a device has, however, the disadvantage that said auxiliary pressure rollers tend to damage the fixing rolls to which they are brought in contacting relation, whereby scratch marks are generated on the image record carrier.
A pressure fixing device of another configuration is known wherein fixing rolls are arranged in a slightly crossed manner with respect to each other. The diameters of said rolls crossing angle. With the bending deformation of said rolls under the application of a load being taken into consideration, it is, however, of disadvantage for such a device that unnecessary stress exerted on a record carrier develops creases and cracks on the record carrier, since the rotating directions of said rolls and forwarding direction of said record carrier are inconsistent by virtue of the non-parallel arrangement of said rolls.
The fixing rolls used in the above-mentioned devices are supported each at both extremities thereof by means of bearings and are subject to a total amount of the load. Thus it has been found that the bearings tend to be damaged due to the excess lead generated when an image record carrier is thick or is fed simultaneously with two or more sheets being overlapped, specifically in the case of fixing operations conducted under an elevated total load.